configure: add --with-git option

Add MP_TOOL_PATH(GIT, [git]) to configure.ac so that users can supply
an alternate git binary via --with-git=PATH at configure time, matching
the pattern used for rsync, openssl, cvs, and other tools.

Fixes: https://trac.macports.org/ticket/72451

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

configure

@@ -844,6 +844,7 @@ enable_flag_sanitization
 with_bsdmake
 with_bzip2_bin
 with_cvs
+with_git
 with_gnumake
 with_gnutar
 with_lzma
@@ -1518,6 +1519,7 @@ Optional Packages:
   --with-bsdmake=PATH     path to alternate bsdmake/pmake command
   --with-bzip2_bin=PATH   path to alternate bzip2 command
   --with-cvs=PATH         path to alternate cvs command
+  --with-git=PATH         path to alternate git command
   --with-gnumake=PATH     path to alternate gnumake command
   --with-gnutar=PATH      path to alternate gnutar command
   --with-lzma=PATH        path to alternate lzma command
@@ -4151,6 +4153,13 @@ then :
 fi
 
 
+# Check whether --with-git was given.
+if test ${with_git+y}
+then :
+  withval=$with_git; GIT=$withval
+fi
+
+
 # Check whether --with-gnumake was given.
 if test ${with_gnumake+y}
 then :

configure.ac

@@ -122,6 +122,7 @@ dnl named with a _BIN suffix here.
 MP_TOOL_PATH(BSDMAKE, [bsdmake/pmake])
 MP_TOOL_PATH(BZIP2_BIN, [bzip2])
 MP_TOOL_PATH(CVS, [cvs])
+MP_TOOL_PATH(GIT, [git])
 MP_TOOL_PATH(GNUMAKE, [gnumake])
 MP_TOOL_PATH(GNUTAR, [gnutar])
 MP_TOOL_PATH(LZMA, [lzma])